1. Field of the Invention
The present invention relates to a liquid crystal display (LCD) device and a method for fabricating the same. More particularly, the present invention relates to a thin film transistor array substrate having spacers formed without sacrificing an aperture ratio.
2. Discussion of the Related Art
Generally, liquid crystal displays (LCDs) control light transmittance of liquid crystal material by using an electric field to display a picture. The liquid crystal display, in which a common electrode formed on an upper substrate and a pixel electrode formed on a lower substrate are arranged with the two electrodes facing each other, drives a liquid crystal by an electric field formed between the common electrode and the pixel electrode.
The liquid crystal display has a thin film transistor array substrate (a lower substrate) and a color filter array substrate (an upper substrate) with the two substrates facing each other and being bonded together, spacers for uniformly maintaining a cell gap between the two substrates and a liquid crystal in the space provided by the spacers.
The thin film transistor array substrate includes a plurality of signal lines, a plurality of thin film transistors, and an alignment film applied for a liquid crystal alignment thereon. The color filter array substrate includes a color filter for representing colors, a black matrix for preventing a light leakage and an alignment film applied for a liquid crystal alignment thereon.
The spacers are classified into ball spacers formed by a scattering method and pattern spacers formed by a photolithography technique.
The ball spacers are scattered on a substrate using a scatter to maintain the cell gap between the upper and lower substrates. However, it is difficult to uniformly scatter the ball spacers. Further, the ball spacers move around between the upper and lower substrates, which results in a ripple phenomenon.
The pattern spacers are formed on a substrate with a pattern by a photolithography technique such that they are fixed at a specific location to maintain the cell gap between the upper and lower substrates. However, because the pattern spacers are formed by the photolithography technique, an additional mask process is required. Moreover, when forming the pattern spacers by a photolithography technique, only a small fraction of the spacer material is actually used to form the pattern spacers, and most of the spacer material, more than 95%, is removed from the substrate, thereby increasing the production cost.
In order to solve these problems, a thin film transistor array substrate having spacers formed by an ink-jet device has been suggested.
FIG. 1 is a plan view illustrating a related art thin film transistor array substrate in which spacers are formed by an ink-jet device, and FIG. 2 is a sectional view illustrating the thin film transistor array substrate taken along the line I-I′ in FIG. 1.
Referring to FIGS. 1 and 2, the related art thin film transistor array substrate includes a gate line 2 and a data line 4 formed on a lower substrate 1 in such a manner to cross each other, a thin film transistor 30 formed at each crossing, a pixel electrode 22 in a pixel region 34 defined by the crossing, a storage capacitor 28 formed at an overlap between the gate line 2 and a storage electrode 24, and a spacer 32 overlapping the storage capacitor 28.
The gate line 2 supplies a gate signal to a gate electrode 6 of the thin film transistor 30. The gate line is formed to have a first width W1 at an area where the gate line 2 and the data line 4 overlap each other with a gate insulating film 12 therebetween, and is formed to have a second width W2, which is wider than the first width W1, at an area between the pixel electrodes 22. That is, the gate line 2 has a relatively narrow width at an area that is overlapped with the data line 4. Thus, the signal interference caused by a coupling between a pixel signal supplied to the data line 4 and a gate signal supplied to the gate line 2 can be reduced.
The data line 4 is formed to have a third width W3 and supplies a pixel signal to the pixel electrode 22 via a drain electrode 10 of the thin film transistor 30.
The thin film transistor 30, in response to the gate signal of the gate line 2, charges the pixel signal of the data line 4 to the pixel electrode 22. To this end, the thin film transistor 30 includes the gate electrode 6 connected to the gate line 2, a source electrode 8 connected to the data line 4, and a drain electrode 10 connected to the pixel electrode 22. The thin film transistor 30 further includes an active layer 14 overlapping the gate electrode 6 with the gate insulating film 12 therebetween and defining a channel between the source electrode 8 and the drain electrode 10. On the active layer 14 is an ohmic contact layer 16 for making an ohmic contact with the source electrode 8 and the drain electrode 10.
The pixel electrode 22, which is connected to the drain electrode 10 of the thin film transistor 30 via a first contact hole 20a passing through a passivation film 18, is formed in the pixel region 34.
Accordingly, an electric field is formed between the pixel electrode 22 to which the pixel signal is supplied via the thin film transistor 30 and the common electrode (not shown) to which a reference voltage is supplied. When such an electric field is applied, the liquid crystal molecules arranged in a predetermined direction between the thin film transistor array substrate and the color filter array substrate rotate due to a dielectric anisotropy of the liquid crystal molecules. As a result, the light transmittance at the pixel region 34 differs in accordance with an amount of the rotation of the liquid crystal molecules, and thereby pictures can be displayed.
The storage capacitor 28 includes the gate line 2, the storage electrode 24 overlapping the gate line 2 with the gate insulating film 12 therebetween, and the pixel electrode 22 connected, via a second contact hole 20b passing through the passivation film 18, to the storage electrode 24. The storage capacitor 28 allows a pixel signal charged in the pixel electrode 22 to be stably maintained until the next pixel signal is charged.
The spacer 32 maintains a cell gap between the thin film transistor array substrate and the color filter array substrate. The spacer 32 is formed by an ink-jet device at a region of the thin film transistor array substrate that is overlapped with a black matrix (not shown) of the color filter array substrate. That is, the spacer 32 is formed to overlap the TFT 30 or the storage capacitor 28 formed on the thin film transistor array substrate.
A method of fabricating the spacer 32 with an ink-jet device will be explained in detail in conjunction with FIGS. 3A to 3C.
As shown in FIG. 3A, an ink-jet device 40 is aligned on the lower substrate 1. A spacer material 33 is then dispensed on the TFT 30 or the storage capacitor 28 of the lower substrate 1 using the aligned ink-jet device 40, as shown in FIG. 3B. That is, when a voltage is applied to a piezoelectric element of an ink-jet head 44, a physical pressure is generated. This physical pressure causes a conduit employed to connect the tank 42 containing the spacer material 33 with a nozzle 46 to contract and relax repeatedly so that the spacer material 33 is dispensed on the lower substrate 1 through the nozzle 46.
The spacer material 33 dispensed on the lower substrate 1 through the nozzle 46 of ink-jet device 40 thereafter undergoes an exposure to a ultraviolet ray radiated from a light source 48 or a firing process as shown in FIG. 3C. In this way, the spacer 32 is fixed on the lower substrate 1 with a predetermined width W and height H.
During the formation of the spacer 32 using the ink-jet device in accordance with the related art, the spacer material 33 of low viscosity experiences the gravity via the nozzle while being dispensed on the substrate 1. Accordingly, the spacer material 33 becomes spread out widely, making it difficult for the spacer 32 to locate at a predetermined position. In other words, the spacer 32 has to be formed at areas on the lower substrate that can be covered by the black matrix of the upper substrate in order not to sacrifice the aperture ratio of the LCD, with those areas including the TFT 30, the storage capacitor 28, the data line 4, and the gate line 2. With the spacer 32 being spread out, the spacer 32 is undesirably formed on the pixel electrode, which is a non-overlapping area with the black matrix, thereby sacrificing the aperture ratio, and the spacer 32 appear as a stain on the pixel electrode 22.